1430
Inorg. Chem. 2001, 40, 1430-1438
Thermodynamic and Kinetic Studies on the Reaction between the Vitamin B12 Derivative β-(N-Methylimidazolyl)cobalamin and N-Methylimidazole: Ligand Displacement at the r Axial Site of Cobalamins Andrew G. Cregan,† Nicola E. Brasch,*,† and Rudi van Eldik*,‡ Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia, and Institute for Inorganic Chemistry, University of Erlangen-Nu¨rnberg, Egerlandstrasse 1, 91058 Erlangen, Germany ReceiVed August 14, 2000 The equilibria and kinetics of substitution of the 5,6-dimethylbenzimidazole at the R site of β-(N-methylimidazolyl)cobalamin by N-methylimidazole have been investigated, and the product, bis(N-methylimidazolyl)cobalamin, has been characterized by visible and 1H NMR spectroscopies. The equilibrium constant for (N-MeIm)Cbl+ + N-MeIm h (N-MeIm)2Cbl+ was determined by 1H NMR spectroscopy (9.6 ( 0.1 M-1, 25.0 °C, I ) 1.5 M (NaClO4)). The observed rate constant for this reaction exhibits an unusual inverse dependence on Nmethylimidazole concentration, and it is proposed that substitution occurs via a base-off solvent-bound intermediate. Activation parameters typical for a dissociative ligand substitution mechanism are reported at two different N-MeImT concentrations, 5.00 × 10-3 M (∆Hq ) 99 ( 2 kJ mol-1, ∆Sq ) 39 ( 5 J mol-1 K-1, ∆Vq ) 15.0 ( 0.7 cm3 mol-1, and 1.00 M (∆Hq ) 109.4 ( 0.8 kJ mol-1, ∆Sq ) 70 ( 3 J mol-1 K-1, ∆V q ) 16.8 ( 1.1 cm3 mol-1). According to the proposed mechanism, these parameters correspond to the aquation of (N-MeIm)2Cbl+ and the ring-opening reaction of the R-DMBI of (N-MeIm)Cbl+ to give the solvent-bound intermediate in both cases, respectively.
Introduction Vitamin B12 and its derivatives are the most biologically important cobalt(III) compounds known and are essential in the nutrition of animals and humans.1 The equatorial sites of vitamin B12 are occupied by a structurally unique corrin ring which is responsible for the exceptionally high lability of the axial sites of vitamin B12 compared to other Co(III) complexes with N-donor equatorial ligands. Typical substitution rate constants for the β site of vitamin B12 are = 103 M-1 s-1, compared to Co(III) porphyrins = 2 M-1 s-1, bis(ethylenediamine) = 10-2 M-1 s-1, bis(dimethylglyoximate) = 10-4 M-1 s-1, (NH3)4 = 10-5 M-1 s-1.2 In the vitamin B12 derivatives, the R axial site is occupied by an intramolecularly bound 5,6-dimethylbenzimidazole (henceforth referred to as DMBI) and the β axial site is typically occupied by a water (aquacobalamin), cyanide (cyanocobalamin ) vitamin B12), 5′-deoxyadenosine (adenosylcobalamin ) coenzyme B12), or methyl group (methylcobalamin). The latter two forms are required for a number of enzyme reactions for which a key step involves homolytic or heterolytic cleavage of the β Co-C bond for the adenosylcobalamindependent isomerases and for the methylcobalamin-dependent methyltransferases.3 Co-C bond cleavage occurs considerably faster in the presence of the enzyme (e.g., for adenosylcobalamin a ca. 1012 †
Australian National University. University of Erlangen-Nu¨rnberg. (1) (a) Dolphin, D., Ed. B12; John Wiley & Sons: New York, 1982. (b) Golding, B. T. Chem. Brit. 1990, 950. (c) Banerjee, R., Ed. Chemistry and Biochemistry of B12; John Wiley & Sons: New York, 1999. (2) Marques, H. M. J. Chem. Soc., Dalton Trans. 1991, 339. (3) Kra¨utler, B.; Kratky, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 167. ‡
rate enhancement;4 for methylcobalamin a ca. 105 rate enhancement5) in comparison to the enzyme-free cofactors in solution. There has been much speculation as to what is responsible for this rate enhancement, and it now appears that a major contribution to Co-C homolysis arises from conformational changes in the enzyme which occur upon substrate binding, leading to a sterically strained adenosyl group and ultimately to cleavage of the Co-C bond.6 Factors that determine the rate of Co-C heterolysis are less well understood, although it has been suggested that electronic factors are important.3 Studies using model complexes of vitamin B12 have shown that the nature of the ligand trans to the Co-C bond can play a role in the kinetic and thermodynamic stability of this bond,7 although until recently this has been regarded as unimportant as it was assumed that the DMBI remained coordinated at the R position during the enzyme reaction. However, the discovery using EPR spectroscopy and X-ray structure analysis that the R-DMBI can be displaced by a histidine residue from the enzyme for both (4) (a) Finke, R. G.; Hay, B. P. Inorg. Chem. 1984, 23, 3041. (b) Hay, B. P.; Finke, R. G. J. Am. Chem. Soc. 1986, 108, 4820 (c) Hay, B. P.; Finke, R. G. Polyhedron, 1988, 7, 1469. (d) Halpern, J. Science 1985, 227, 869. (e) Halpern, J.; Kim, S.-H.; Leung, T. W. J. Am. Chem. Soc. 1984, 106, 8317. (f) Brown, K. L.; Zou, X. J. Inorg. Biochem. 1999, 77, 185. (5) Banerjee, R. V.; Frasca, V.; Ballou, D. P.; Matthews, R. G. Biochemistry 1990, 29, 11101. (6) (a) Mancia, F.; Evans, P. R. Structure 1998, 6, 711. (b) Champloy, F.; Jogl, G.; Reitzer, R.; Buckel, W.; Bothe, H.; Beatrix, B.; Broeker, G.; Michalowicz, A.; Meyer-Klaucke, W.; Kratky, C. J. Am. Chem. Soc. 1999, 121, 11780. (c) Kra¨utler, B.; Keller, W.; Kratky, C. J. Am. Chem. Soc. 1989, 111, 8936. (d) Mancia, F.; Smith, G. A.; Evans, P. R. Biochemistry 1999, 38, 7999. (7) Pratt, J. M. Inorganic Chemistry of Vitamin B12; Academic Press: London, New York, 1972.
10.1021/ic0009268 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/01/2001
Ligand Displacement at the R Axial Site of Cobalamins adenosylcobalamin-dependent isomerases and methylcobalamindependent methyltransferases has opened up a completely new area for consideration,3,8,9 namely the substitution chemistry of the R axial site of cobalamins. There has been considerable interest in the substitution behavior of the β water ligand of aquacobalamin, which is readily substituted by a wide range of nucleophiles (e.g., L ) SCN-, CN-, NO2-, Br-, I-, NCO-, SO32-, pyridine, imidazole, S2O32-, N3-, thiourea, glycine, NH2R, Fe(CN)64-).2,10-13 It is generally agreed that this occurs via a dissociative interchange mechanism; the experimental evidence for this includes the observed rate constant being essentially independent of the nature of the entering ligand for a wide range of ligands, kinetic evidence for ion-pair formation in a few special cases11 and positive volumes of activation.11a,b,12 However, as far as we are aware, the only ligand known to be capable of substituting the DMBI at the R axial site of cobalamins (apart from (R-DMBI)Co cleavage resulting from protonation of the N1 nitrogen of R-DMBI) is cyanide (adenosylcobalamin,14a-c cyanocobalamin,14d (oxocarbonyl)cobalamins,14e,f benzylcobalamin,14g other cobalamins14h), with the majority of authors probably regarding this site as being inert in the absence of cyanide and acid.7,10b During their experiments to determine formation constants for imidazolylcobalamins by visible spectroscopy, Marques et al.15 observed the presence of a new species at higher imidazole (8) (a) Stupperich, E.; Eisinger, H. J.; Albracht, S. P. J. Eur. J. Biochem. 1990, 193, 105. (b) Drennan, C. L.; Huang, S.; Drummond, J. T.; Matthews, R. G.; Ludwig, M. L. Science 1994, 266, 1669. (c) Drennan, C. L.; Matthews, R. G.; Ludwig, M. L. Curr. Opin. Struct. Biol. 1994, 4, 919. (d) Padmakumar, R.; Taoka, S.; Padmakumar. R.; Banerjee, R. J. Am. Chem. Soc. 1995, 117, 7033. (e) Zelder, O.; Beatrix, B.; Kroll, F.; Buckel, W. FEBS Lett. 1995, 369, 252. (f) Mancia, F.; Keep, N. H.; Nakagawa, A.; Leadlay, P. F.; McSweeney, S.; Rasmussen, B.; Bo¨secke, P.; Diat, O.; Evans, P. R. Structure 1996, 4, 339. (g) Ludwig, M. L.; Drennan, C. L.; Matthews, R. G. Structure 1996, 4, 505. (h) Reitzer, R.; Gruber, K.; Jogl, G.; Wagner, U. G.; Bothe, H.; Buckel, W.; Kratky, C. Structure 1999, 7, 891. (9) Rates of homolytic and heterolytic cleavage of the Co-C bond of 5′-deoxyadenosylcobinamide in the presence of exogenous bases which bind trans to the 5′-Ado group have been examined: (a) Garr, C. D.; Sirovatka, J. M.; Finke, R. G. J. Am. Chem. Soc. 1996, 118, 11142. (b) Sirovatka, J. M.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 3057. (c) Sirovatka, J. M.; Finke, R. G. Inorg. Chem. 1999, 38, 1697. (10) (a) Marques, H. M.; Munro, O. Q.; Cumming, B. M.; de Nysschen, C. J. Chem. Soc., Dalton Trans. 1994, 297. (b) Thusius, D. J. Am. Chem. Soc. 1971, 93, 2629. (c) Marques, H. M. J. Chem. Soc., Dalton Trans. 1991, 1437. (d) Marques, H. M. S.-Afr. Tydskr. Chem. 1991, 44, 114. (e) Marques, H. M.; Bradley, J. C.; Campbell, L. A. J. Chem. Soc. Dalton Trans. 1992, 2019. (f) Marques, H. M.; Knapton, L. J. Chem. Soc., Dalton Trans. 1997, 3827. (11) (a) Prinsloo, F. F.; Meier, M.; van Eldik, R. Inorg. Chem. 1994, 33, 900. (b) Stochel, G.; van Eldik, R. Inorg. Chem. 1990, 29, 2075. (c) Saturation kinetics were also observed in ref 10a,e; however, these results are now questionable since KCl was used to maintain constant ionic strength.10f,12c (12) (a) Marques, H. M.; Breet, E. L. J.; Prinsloo, F. F. J. Chem. Soc., Dalton Trans. 1991, 2941. (b) Stochel, G.; van Eldik, R.; Kunkely, H.; Vogler, A. Inorg. Chem. 1989, 28, 4314. (c) Prinsloo, F. F.; Breet, E. L. J.; van Eldik, R. J. Chem. Soc., Dalton Trans. 1995, 685. (d) Hasinoff, B. B. Can. J. Chem. 1974, 52, 910. (e) Meier, M.; van Eldik, R. Inorg. Chem. 1993, 32, 2635. (13) Marques, H. M.; Egan, T. J.; Marsh, J. H.; Mellor, J. R.; Munro, O. Q. Inorg. Chim. Acta 1989, 166, 249. (14) (a) Rudakova, I. P.; Pospelova, T. A.; Borodulina-Shvets, V. I.; Kurganov, B. I.; Yurkevich, A. M. J. Organomet. Chem. 1973, 61, 389. (b) Brasch, N. E.; Hamza, M. S. A.; van Eldik, R. Inorg. Chem. 1997, 36, 3216. (c) Haupt, R. J.; Brasch, N. E. Inorg. Chem. 2000, 39, 5469. (d) Reenstra, W. W.; Jencks, W. P. J. Am. Chem. Soc. 1979, 101, 5780. (e) Reenstra, W. W.; Abeles, R. H.; Jencks, W. P. J. Am. Chem. Soc. 1982, 104, 1016. (f) Reenstra, W. W.; Abeles, R. H.; Jencks, W. P. Fed. Proc. 1976, 35, 951. (g) Nome, F.; Rezende, M. C.; Sabo´ia, C. M.; Clemente da Silva, A. Can. J. Chem. 1987, 65, 2095. (h) Brown, K. L. J. Am. Chem. Soc. 1987, 109, 2277. (15) Marques, H. M.; Marsh, J. H.; Mellor, J. R.; Munro, O. Q. Inorg. Chim. Acta 1990, 170, 259.
Inorganic Chemistry, Vol. 40, No. 7, 2001 1431
Figure 1. 1H NMR spectra of the aromatic region for equilibrated solutions of HOCbl‚HCl (ca. 3.0 × 10-3 M) and varying concentrations of N-MeImT: 3.00 × 10-3 (a), 5.00 × 10-2 (b), 0.100 (c), or 0.250 M (d) (25.0 °C, in D2O, 0.100 M TAPS buffer, pD 8.51). The signals at 5.85, 6.14, 6.30(d) 6.43, 6.68, 6.85, 7.06, and 7.23 ppm in spectrum a can be attributed to (N-MeIm)Cbl+. Increasing the N-MeImT concentration (b f d) results in the decrease in the intensity of the (N-MeIm)Cbl+ signals and the concurrent appearance and increase in intensity of signals attributable to (N-MeIm)2Cbl+. Signal assignments are given in Table 1.
concentrations which they were able to separate by HPLC and suggested that an imidazole had displaced the R-DMBI of the imidazolylcobalamins. In light of the evidence demonstrating that histidine can displace the R-DMBI ligand during the B12dependent enzyme reactions, it seems reasonable that this position could also be labile in the absence of the enzyme and that substitution studies on this reaction could be used as a means to increase our understanding of factors accompanying substitution of the R-DMBI by a histidine-type ligand. The paper presents visible and 1H NMR spectroscopic data that show that the R-DMBI ligand of β-(N-methylimidazolyl)cobalamin (henceforth referred to as (N-methylimidazolyl)cobalamin) can be readily displaced by N-methylimidazole under biological pH conditions, thus demonstrating that substitution of the R-DMBI of a cobalamin by a histidine type ligand can also occur in the absence of the enzyme. Equilibria and kinetic data for this reaction have been obtained for a range of pH conditions (pD 6.5-9.5 and pH 5.5-9.5, respectively). Kinetic studies reveal a rather interesting mechanism for this substitution process. Results Characterization of (N-MeIm)Cbl+ and (N-MeIm)2Cbl+. Figure 1a gives the 1H NMR spectrum of the aromatic region16 of a ca. 1:1 mixture of HOCbl‚HCl (H2OCbl+ and HOCbl; pKa(H2OCbl+) ) 8.13 (see below)) and N-MeImT (N-MeImT ) N-MeIm + N-MeImH+) at pD 8.51 ca. 2 h after mixing. The spectrum is very different from that of the starting materials (HOCbl/H2OCbl+ and N-MeIm/N-MeImH+ have signals in the aromatic region at 6.19, 6.23(d), 6.44, 6.58, and 7.16 ppm and 7.13, 7.21, and 7.89 ppm, respectively (25.0 °C, pD 8.51, I ) 1.5 M (NaClO4)), and corresponds to that of (N-MeIm)Cbl+. Eight signals at 5.85, 6.14, 6.30(d), 6.43, 6.68, 6.85, 7.06, and (16) A recent paper demonstrates how useful the aromatic region of the 1H NMR spectrum can be for identifying and quantifying cobalamins in solution; see: Brasch, N. E.; Finke, R. G. J. Inorg. Biochem. 1999, 73, 215.
1432 Inorganic Chemistry, Vol. 40, No. 7, 2001
Cregan et al.
Table 1. 1H NMR Assignments of the Protons in the Aromatic Region for (N-MeIm)Cbl+ and (N-MeIm)2Cbl+. (N-MeIm)Cbl+ a,b
(N-MeIm)2Cbl+ a-c
signal (ppm)
assignment
signal (ppm)d
assignment
5.85 6.14 6.30 (d) 6.43 6.68 6.85 7.06 7.23
β-A5 C10 R1 β-A2 B4 β-A4 B2 B7
5.67 5.78 6.05 6.32 6.41 (d) 6.43 6.77 6.81 7.43 7.48 8.35
A5e A5f C10 A2f R1 A2e A4f A4e B4 or B7 B4 or B7 B2
a
b
See Scheme 1 for coordinated N-MeIm labeling scheme. See ref 17 for cobalamin labeling scheme. c Free (uncoordinated) N-MeImT signals observed at 7.07, 7.20, and 7.85 ppm. d From spectrum 1b. e These signals belong to the same N-MeIm ligand (R or β). f These signals belong to the same N-MeIm ligand (R or β).
Scheme 1. Labeling Scheme for Coordinated N-MeIm Ligands
7.23 ppm are observed in the aromatic region, as expected for (N-MeIm)Cbl+, corresponding to the B2, B4, B7, and R1 signals of the nucleotide, the C10 signal of the corrin ring and the three aromatic signals of the β-coordinated N-MeIm (A2, A4, A5 (see Table 1 for labeling schemes)). The signal assignments are given in Table 1; assignment was possible with the additional information provided by HMQC and HMBC experiments. At longer times the intensity of the β-A2 proton of (N-MeIm)Cbl+ gradually decreases due to relatively rapid proton exchange between this proton and solvent.18 Both equilibria and rate constants have been previously reported for the reaction between H2OCbl+ and N-MeIm to form (N-MeIm)Cbl+.13,15,19 Figure 1b gives the 1H NMR spectrum of HOCbl‚HCl in 5.00 × 10-2 M N-MeImT (pD 8.51, 25.0 °C, I ) 1.5 M (NaClO4)). A whole set of new signals are observed and correspond to the 11 expected signals of (N-MeIm)2Cbl+; five from the nucleotide and corrin ring, three from the β-coordinated N-MeIm, and three from the R-coordinated N-MeIm, the latter having displaced the intramolecularly coordinated DMBI from the R axial site. Signals attributable to (N-MeIm)Cbl+ are also visible. From Figure 1b-d it can be seen that the (N-MeIm)2Cbl+ signals in the aromatic region increase in intensity as the N-MeImT concentration increases (with a corresponding decrease in the (NMeIm)Cbl+ signals), since more (N-MeIm)2Cbl+ is formed at higher N-MeImT concentrations. Signal assignments for (N-MeIm)2Cbl+ in the aromatic region are summarized in Table 1. Figure 2 gives visible spectra for (N-MeIm)Cbl+ (λmax ((1 nm) ) 359, 414, 512 (shoulder) and 540 nm; [Cbl]T ) 4.40 × (17) Calafat, A. M.; Marzilli, L. G. J. Am. Chem. Soc. 1993, 115, 9182. (18) (a) Vaughan, J. D.; Mughrabi, Z.; Wu, E. C. J. Org. Chem. 1970, 35, 1141. (b) Harris, T. M.; Randell, J. C. Chem. Ind. 1965, 1728. (c) Staab, H. A.; Wu, M. T.; Mannschreck, A.; Schwalbach, G. Tetrahedron Lett. 1964, 15, 845. (d) Gillespie, R. J.; Grimison, A.; Ridd, J. H.; White, R. F. M. J. Chem. Soc. 1958, 3228. (19) Eilbeck, W. J.; West, M. S. J. Chem. Soc., Dalton Trans. 1976, 274.
Figure 2. Visible absorbance spectra of (N-MeIm)Cbl+ (s, [Cbl]T ) 4.40 × 10-5 M, N-MeImT ) 1.50 × 10-3 M, λmax (, M-1 cm-1) ) 359 (2.89 × 104), 414 (4.20 × 103), 512 (shoulder, 8.77 × 103), 540 (9.95 × 103) nm) and (N-MeIm)2Cbl+ (- - -, [Cbl]T ) 4.40 × 10-5 M, N-MeImT ) 2.20 M, λmax (, M-1 cm-1) ) 358 (3.40 × 104), 411 (3.96 × 103), 511 (shoulder, 9.66 × 103), 541 (1.06 × 104) nm). Errors in the extinction coefficients are estimated to be (4%. Both spectra are recorded at 25.0 °C in 0.100 M TAPS buffer, pH 8.50, I ) 1.5 M (NaClO4).
10-5 M, 0.100 M TAPS buffer, pH 8.50, 25.0 °C) and (N-MeIm)2Cbl+ (λmax ((1 nm) ) 358, 411, 511 (shoulder) and 541 nm; [Cbl]T ) 4.40 × 10-5 M, 0.100 M TAPS buffer, pH 8.50, 25.0 °C). Both spectra are significantly different from that of HOCbl/H2OCbl+ at the same conditions (λmax ) 356, 414, 515(s) and 532 nm; [Cbl]T ) 4.40 × 10-5 M, 0.100 M TAPS buffer, pH 8.50, 25.0 °C, I ) 1.5 M (NaClO4)). Wavelength maxima of (N-MeIm)Cbl+ and (N-MeIm)2Cbl+ are very similar, which is to be expected since the complexes essentially only differ by the type of imidazole ligand they have coordinated to the R axial site. Similar visible spectra were observed by Marques and co-workers.15 A FAB-MS spectrum (positive ion mode) was also obtained for a sample of (N-MeIm)2Cbl+‚Cl- precipitated from a solution of HOCbl‚HCl in ca. 2 M N-MeImT (pH 8.5). A small peak was observed at m/z ) 1492.9 (calculated m/z for (NMeIm)2Cbl+ (C70H100O14N17PCo) ) 1492.7), confirming that (N-MeIm)2Cbl+ is formed at high N-MeImT concentrations. Acid Dissociation Constants. The acid dissociation constants of H2OCbl+ and N-MeImH+ at 25.0 °C and I ) 1.5 M (NaClO4) were determined by potentiometric titration to be (7.38 ( 0.16) × 10-9 M (pKa ) 8.13 ( 0.01) and (2.91 ( 0.10) × 10-8 M (pKa ) 7.54 ( 0.02), respectively. Both values are the mean of two separate determinations. The value for H2OCbl+ is in good agreement with previous values (at 25 °C): 8.1020a (I ) 1.0 M, KCl), 8.114d (I ) 1.0 M, KCl), 8.0920b (I ) 0.5 M, NaNO3), 7.9312a (I ) 1.0 M, KCl), 7.820c (0.05 M phosphate buffer) and 7.6220d (I ) 0.1 M, KNO3). Interestingly, the presence of NaClO4 (I ) 1.5 M) appears to significantly increase the pKa of N-MeImH+; reported literature values (at 25 °C) include 7.2113 (I ) 1.0 M (KCl)), 7.221a (I ) 0.1 M, phosphate buffer), 7.2521b (I ) 0.1 M, KCl), 7.2021c (I ) 0.15 M, KCl), 7.1921d (I ) 0.5 M, KNO3), 7.2021e (I ) 1.0 M, (CH3)4N+Cl-), 7.0519 (I ) 0.1 M, KNO3) and 7.2021f (I ) 0.15 M, KNO3). Equilibrium Constants. As shown in Scheme 2, when N-MeIm is reacted with H2OCbl+, both (N-MeIm)Cbl+ and (20) (a) Marques, H. M.; Brown, K. L.; Jacobsen, D. W. J. Biol. Chem. 1988, 263, 12378. (b) Marques, H. M.; Knapton, L. J. Chem. Soc., Dalton Trans. 1997, 3827. (c) Hayward, G. C.; Hill, H. A. O.; Pratt, J. M.; Vanston, N. J.; Williams, R. J. P. J. Chem. Soc. 1965, 6485. (d) Eilbeck, W. J.; West, M. S.; Owen, Y. E. J. Chem. Soc., Dalton Trans. 1974, 2205.
Ligand Displacement at the R Axial Site of Cobalamins
Inorganic Chemistry, Vol. 40, No. 7, 2001 1433
Scheme 2
(N-MeIm)2Cbl+ form, the proportions of each cobalamin depending on the pH of the solution and the concentration of N-MeIm. It was therefore necessary to have a complete understanding of the equilibria (K1) and kinetics (k1, k-1) for the formation of (N-MeIm)Cbl+ from H2OCbl+ and N-MeIm prior to investigating the reaction between (N-MeIm)Cbl+ and N-MeIm, so that the latter reaction could be studied under conditions where there would be no interference from the former reaction. The equilibrium constant, Kobs(1) for the equilibrium Kobs(1)
HOCbl/H2OCbl+ + N-MeImT y\z (N-MeIm)Cbl+
(1)
(N-MeImT ) N-MeIm + N-MeImH+) was determined at four pH conditions (pH 6.51, 6.99, 7.52, 7.98) by measuring the absorbance of equilibrated solutions containing a fixed amount of HOCbl‚HCl (ca. 5 × 10-5 M, exact concentration known) and a varying amount of N-MeImT (0-2.34 × 10-3 M) by visible spectroscopy (25.0 °C, I ) 1.5 M (NaClO4)). Figure 3 gives a plot of observed absorbance (Aobs) at 358 nm versus total N-MeIm concentration for the data obtained at pH 6.51. Similar plots at the other pH conditions are given in Figures A-C, Supporting Information. The fitted curve represents the best fit of the data to eq 2, which is derived in the Supporting Information.
Aobs ) [A(N-MeIm)Cbl+(X - Y)/2Kobs(1)[Cbl]T] + [([Cbl]T (X - Y)/2Kobs(1))AH2OCbl+/[Cbl]T] (2) where X ) Kobs(1)[Cbl]T + Kobs(1)[N-MeIm]C + 1, Y ) [X2 4(Kobs(1))2[Cbl]T[N-MeIm]C]1/2, [N-MeIm]C ) [(N-MeIm)Cbl+] + [N-MeIm] + [N-MeImH+], [Cbl]T ) [H2OCbl+] + [HOCbl] + [(N-MeIm)Cbl+], A(N-MeIm)Cbl+ ) absorbance of (N-MeIm)Cbl+, and AH2OCbl+ ) absorbance of H2OCbl+. The equilibrium constant Kobs(1) was found to be (1.28 ( 0.06) × 104, (3.08 ( 0.14) × 104, (5.41 ( 0.35) × 104, and (6.26 ( 0.35) × 104 M-1 at pH 6.51, 6.99, 7.52, and 7.98, respectively. Assuming that only H2OCbl+ and N-MeIm react to give (NMeIm)Cbl+ (i.e., that HOCbl and N-MeImH+ do not react), corresponding K1 values can be calculated, where K1 is defined by K1
H2OCbl+ + N-MeIm y\z (N-MeIm)Cbl+
(3)
(21) (a) Hamza, M. S. A.; Pratt, J. M. J. Chem. Soc., Dalton Trans. 1994, 1367. (b) Catala´n, J.; Claramont, R. M.; Elguero, J.; Laynez, J.; Mene´ndez, M.; Anvia, F.; Quian, J. H.; Taagepera, M.; Taft, R. W. J. Am. Chem. Soc. 1988, 110, 4105. (c) Paiva, A. C. M.; Juliano, L.; Boschcov, P. J. Am. Chem. Soc. 1976, 98, 7645. (d) Lenarcik, B.; Barszcz, B. Rocz. Chem. 1977, 51, 1849. (e) Oakenfull, D. G.; Jencks, W. P. J. Am. Chem. Soc. 1971, 93, 178. (f) Li, N. C.; White, J. M.; Doody, E. J. Am. Chem. Soc. 1954, 76, 6219.
Figure 3. Plot of observed absorbance (Aobs) at 358 nm versus total N-MeIm concentration for equilibrated solutions of HOCbl‚HCl (4.60 × 10-5 M) and varying concentrations of N-MeImT (0-2.34 × 10-3 M); 0.100 M BIS-TRIS buffer, pH 6.51, I ) 1.5 M (NaClO4). The fitted curve represents the best fit of the data to eq 2; fixing [CblT] ) 4.60 × 10-5 M gives Kobs(1) ) (1.28 ( 0.06) × 104 M-1, AH2OCbl+ ) 1.089 ( 0.002, and A(N-MeIm)Cbl+ ) 1.386 ( 0.002.
Using potentiometrically determined values of pKa(H2OCbl+) ) 8.13 ( 0.01 (25.0 °C, I ) 1.5 M (NaClO4)) and pKa(NMeImH+) ) 7.54 ( 0.02 (25.0 °C, I ) 1.5 M (NaClO4)), a mean value of K1 ) (1.47 ( 0.07) × 105 M-1 is obtained. The excellent agreement between the four K1 values obtained at different pH conditions demonstrates that the assumptions that HOCbl and N-MeImH+ do not react are valid for this system. Attempts were made to determine the equilibrium constant Kobs(2) for Kobs(2)
(N-MeIm)Cbl+ + N-MeImT y\z (N-MeIm)2Cbl+ (4) by visible spectroscopy. There was, however, considerable scatter in the plots of observed absorbance versus N-MeIm concentration, since the visible spectra of (N-MeIm)Cbl+ and (N-MeIm)2Cbl+ are very similar (Figure 2). Values of Kobs(2) were therefore determined in D2O at seven pD conditions by 1H NMR spectroscopy by comparing the signal area of one signal from each of the two complexes (5.88 ( 0.04 ppm for (N-MeIm)Cbl+, 5.71 ( 0.05 ppm for (N-MeIm)2Cbl+)) at each pD condition for a series of solutions with varying N-MeImT concentrations (5.00 × 10-2 - 1.20 M); see Table A in the Supporting Information. The data are summarized in Figure 4. Note that under all conditions formation of (N-MeIm)Cbl+ from (H2OCbl+ + N-MeIm) is complete; that is, no H2OCbl+ (or HOCbl) is present. Assuming only N-MeIm reacts with (NMeIm)Cbl+ to form (N-MeIm)2Cbl+ (i.e., N-MeImH+ cannot coordinate to (N-MeIm)Cbl+), K2 can be calculated, where K2 corresponds to the equilibrium K2
(N-MeIm)Cbl+ + N-MeIm y\z (N-MeIm)2Cbl+
(5)
1434 Inorganic Chemistry, Vol. 40, No. 7, 2001
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Figure 4. Plot of the observed equilibrium constant Kobs(2) versus pD, where Kobs(2) refers to the equilibrium given in eq 4. The fitted curve represents the best fit of the data to eq 6; fixing Ka(N-MeImH+) ) 2.91 × 10-8 M gives K2 ) 9.6 ( 0.1 M-1.
The fitted curve in Figure 4 represents the best fit of the data to the equation +
+
Kobs(2) ) (Ka(N-MeImH )K2)/(Ka(N-MeImH ) + log10-1(-pD)) (6) Fixing Ka(N-MeImH+) to 2.91 × 10-8 M (pKa ) 7.54) gives K2 ) 9.6 ( 0.1 M-1. The excellent fit of the data shows that the assumption that only N-MeIm can react with (N-MeIm)Cbl+ is valid. Kinetics. The concentration dependence of the observed rate constant for the reaction kobs(1)
HOCbl/H2OCbl+ + N-MeImT 98 (N-MeIm)Cbl+
(7)
was examined by visible spectroscopy (358 nm) at six different pH conditions, pH 6.01, 7.01, 7.30, 8.00, 8.50, and 9.00 (25.0 °C, I ) 1.5 M (NaClO4)). Pseudo-first-order conditions with respect to N-MeImT were used, and N-MeIm concentrations were low enough (< 5.00 × 10-3 M) so very little (NMeIm)2Cbl+ was formed, usually much less than 4%. For each pH condition the plot of the observed rate constant, kobs(1), versus [N-MeIm]T was linear, and intercepted the origin (Figures D-I, Supporting Information), meaning that the reaction is irreversible under all experimental conditions. Values for kobs(1) were obtained from the slopes of the plots. Since it has previously been shown that only H2OCbl+ and N-MeIm react to form (NMeIm)Cbl+ 13 k1
H2OCbl+ + N-MeIm 98 (N-MeIm)Cbl+
(8)
the pH-independent rate constant k1 can be obtained by allowing for the fraction of H2OCbl+ and N-MeIm present at each pH condition. Values of k1 ) 68.9, 80.2, 74.4, 79.5, 85.1, and 85.1 M-1 s-1 were obtained at pH 6.01, 7.01, 7.30, 8.00, 8.50, and 9.00, respectively, giving a mean k1 ) 79 ( 6 M-1 s-1. The concentration dependence of the observed rate constant, kobs(2) for the reaction kobs(2)
(N-MeIm)Cbl+ + N-MeIm {\} (N-MeIm)2Cbl+ (9) was measured at pH 8.50 by visible spectroscopy (358 nm, 25.0 °C, I ) 1.5 M (NaClO4)).The results are summarized in Figure 5 and show that kobs(2) decreases with increasing N-MeIm.
Figure 5. Plot of observed rate constant, kobs(2), versus [N-MeIm] for the reaction (N-MeIm)Cbl+ + N-MeIm a (N-MeIm)2Cbl+ at pH 8.50 (0.100 M TAPS buffer, 25.0 °C, I ) 1.5 M (NaClO4)). The curve represents the best fit of the data to eq 11 in the text. Fixing K2 ) 9.6 M-1 gives k3 ) (2.08 ( 0.02) × 10-3 s-1 and k-4 ) (4.00 ( 0.04) × 10-3 s-1.
Results of kinetic scanning experiments (from 300 to 600 nm) confirmed that the same reaction is being followed over the entire concentration range, since the reaction isosbestic points are independent of concentration. For example, at pH 8.50, isosbestic points were observed at 335 ( 1, 366 ( 1, 404 ( 1, 410 ( 1, 448 ( 3 and 563 ( 1 nm for both 0.800 and 5.00 × 10-3 M N-MeImT (25.0 °C, I ) 1.5 M (NaClO4)). The concentration dependence of kobs(2) was also examined at pH 7.00 ([N-MeIm]T ) 5.00 × 10-3 - 1.20 M ([N-MeIm] ) 1.12 × 10-3 - 0.269 M). The results are summarized in Figure J, Supporting Information. Much smaller changes (e 15%) in kobs(2) were observed at this pH condition, although once again it is apparent that kobs(2) decreases with increasing N-MeIm concentration for [N-MeIm] < 0.18 M. For 0.180.27 M N-MeIm, kobs(2) was found to increase ca. 5%. It is likely that this small increase arises from medium effects,22 since above 0.18 M N-MeIm, N-MeImH+ inevitably contributes significantly (> 40%; [N-MeImH+] > 0.62 M) to the cationic component of the ionic strength. For the experiments shown in Figure 5 (pH 8.50), N-MeImH+ always contributes e8% to the cationic component of the ionic strength. Figure 6a shows the behavior of kobs(2) as a function of pH (0.800 M N-MeImT, 25.0 °C, I ) 1.5 M (NaClO4)). The pH dependence of the observed rate constant at 5.00 × 10-3 M N-MeImT (25.0 °C, I ) 1.5 M (NaClO4)) was also examined at pH 5.50, 7.00, and 8.50 and found to be pH-independent (kobs(2) ) (3.93 ( 0.06) × 10-3 s-1; see Table B in the Supporting Information). Activation Parameters. The temperature dependence of kobs was examined at pH 8.50 for two N-MeImT conditions, 5.00 × 10-3 (18-40 °C) and 1.00 M (10-40 °C) in 0.100 M TAPS buffer at I ) 1.5 M (NaClO4). The reason for choosing these two conditions will become clearer in the Discussion, but basically, conditions have been chosen where the rate constants either correspond to aquation of (N-MeIm)2Cbl+ or ring-opening in (N-MeIm)Cbl+ to give an intermediate. Activation parameters were obtained from Eyring plots, giving ∆Hq ) 99 ( 2 kJ mol-1, ∆Sq ) 39 ( 5 J mol-1 K-1 and ∆Hq ) 109.4 ( 0.8 kJ mol-1, ∆Sq ) 70 ( 3 J mol-1 K-1 for 5.00 × 10-3 and 1.00 M N-MeImT, respectively. The pressure dependence of the observed rate constant at 15.0 °C was also investigated and plots (22) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd ed.; VCH: Weinheim, Germany, 1991; p 116.
Ligand Displacement at the R Axial Site of Cobalamins
Inorganic Chemistry, Vol. 40, No. 7, 2001 1435 always irreversible under all experimental conditions in which the equilibria or kinetics of the reaction of (N-MeIm)Cbl+ with N-MeIm to give (N-MeIm)2Cbl+ were studied; that is, no H2OCbl+ or HOCbl are present at equilibrium. Our value of k1 ) 79 ( 6 M-1 s-1, 25.0 °C, I ) 1.5 M (NaClO4)) differs significantly from that reported in KCl (k1 ) 16.6 ( 0.3 M-1 s-1, 25.0 °C, I ) 1.0 M13); hence, it appears that the formation of chlorocobalamin hinders the rate of formation of (N-MeIm)Cbl+ from H2OCbl+ and N-MeIm, as might be expected.10f,12c The equilibrium constant K2 (eq 5) was determined by 1H NMR spectroscopy to be 9.6 ( 0.1 M-1, meaning that significant concentrations (>5%) of (N-MeIm)2Cbl+ exist in solution for N-MeIm g 6 mM. Marques and co-workers estimated K2 to be 1.6 M-1 using HPLC;15 however, this value is less reliable since once again KCl was used to maintain ionic strength (I ) 1.0 M). The most interesting result is the decrease in the observed rate constant, kobs(2) for the reaction (N-MeIm)Cbl+ + N-MeIm h (N-MeIm)2Cbl+ at pH 8.50 (Figure 5) with increasing N-MeIm concentration. While rather unusual, there are several cases of this reported in the literature.23 The results are consistent with a mechanism of the type
Figure 6. (a) Plot of observed rate, kobs(2) versus pH for the reaction (N-MeIm)Cbl+ + N-MeIm a (N-MeIm)2Cbl+ (0.800 M N-MeImT, 0.100 M buffer (except for the data at pH 9.49), 25.0 °C, I ) 1.5 M (NaClO4)). (b) Plot of kobs(2) versus [N-MeIm] (X) for the data given in (a). The data from Figure 5 are also shown on the plot (O).
of ln kobs(2) versus pressure gave ∆Vq ) 15.0 ( 0.7 and 16.8 ( 1.1 cm3 mol-1 at 5.00 × 10-3 and 1.00 M N-MeImT, respectively (pH 8.50, 0.100 M TAPS buffer, I ) 1.5 M (NaClO4); Figures K and L, Supporting Information). Discussion Two things were required prior to studying the reaction between (N-MeIm)Cbl+ and N-MeIm to form (N-MeIm)2Cbl+. First, the existence of (N-MeIm)2Cbl+ in solutions containing high concentrations of N-MeIm needed to be demonstrated, which was achieved by 1H NMR spectroscopy (Figure 1) and FAB-MS. In addition, both the equilibrium (K1) and rate constant (k1) for the formation of (N-MeIm)Cbl+ from H2OCbl+ and N-MeIm were determined, so that the reaction of interest could be studied under conditions where the former reaction would not interfere. Recently, both K1 and k1 were determined using KCl as the supporting electrolyte.13,15 It has since been found, however, that chloride binds to H2OCbl+,10f,12c making these data questionable. In addition, it is useful to re-determine these values at a total ionic strength of 1.5 M using NaClO4 as the supporting electrolyte to allow comparison with other data reported in this paper. K1 was determined by visible spectroscopy as the prior study in KCl had done.15 Our value of K1 ) (1.47 ( 0.07) × 105 M-1 (25.0 °C, I ) 1.5 M (NaClO4)) is larger than the values determined using either KCl or KNO3 as a supporting electrolyte (K1 ) (4.3 ( 0.5) × 104 M-1, 25.0 °C, I ) 1.0 M (KCl);15 K1 ) (2.5 ( 0.50) × 104 M-1, 25 °C, I ) 0.10 M (KNO3)19). Since K1 is so large, a simple calculation shows that eq 1 is
with the associated rate equation
kobs(2) ) (k3k4[N-MeIm] + k-3k-4)/(k-3 + k4[N-MeIm]) (11) ) K2k-4[N-MeIm] + k-4/(1 + K2k-4[N-MeIm]/k3) (12) It can be seen from eq 11 that at high N-MeIm concentrations kobs(2) will reach a limiting value () k3). The observed rate constant also approaches a limiting value at low N-MeIm concentrations () k-4). Hence, kobs(2) decreases with increasing N-MeIm concentration simply because k-4 is larger than k3. The curve superimposed on the data in Figure 5 represents the fit of the data to eq 11, fixing K2 ) 9.6 M-1.24 The best fit gives k3 ) (2.08 ( 0.02) × 10-3 s-1 and k-4 ) (4.00 ( 0.04) × 10-3 s-1. Additional experiments were carried out to investigate the pH dependence of k-4 from measurements of kobs(2) () k-4) for solutions containing 5.00 × 10-3 M N-MeImT at various pH conditions (pH 5.50, 7.00 and 8.50). The results are summarized in Table B of the Supporting Information and demonstrate that k-4 is pH-independent () (3.93 ( 0.06) × 10-3 s-1). A second possible mechanism which also fits the data given in Figure 5 is a “dead end” type of mechanism (23) (a) Toma, H. E.; Malin, J. M.; Giesbrecht, E. Inorg. Chem. 1973, 12, 2084. (b) Malin, J. M.; Toma, H. E.; Giesbrecht, E. J. Chem. Ed. 1977, 54, 385. (c) Byers, W.; Cossham, J. A.; Edwards, J. O.; Gordon, A. T.; Jones, J. G.; Kenny, E. T. P.; Mahmood, A.; McKnight, J.; Sweigart, D. A.; Tondreau, G. A.; Wright, T. Inorg. Chem. 1986, 25, 4767. (d) Byrd, J. E.; Wilmarth, W. K. Inorg. Chim. Act. ReV. 1971, 7. (24) This assumes that K2 is the same in H2O and D2O.
1436 Inorganic Chemistry, Vol. 40, No. 7, 2001
Cregan et al.
with the corresponding rate equation
kobs(2) ) k3/(1 + K[N-MeIm]) + k-3
(14)
Fitting the data gives K ) 15 ( 2 M-1 (k3 ) (1.89 ( 0.05) × 10-3 M-1 s-1, k-3 ) (2.03 ( 0.03) × 10-3 s-1), which is larger than that expected for association between (N-MeIm)Cbl+ and neutral N-MeIm. It is therefore unlikely the reaction is occurring via a mechanism of this type. The pH profile, Figure 6a, also qualitatively supports the concentration profile at pH 8.50 in that once again kobs(2) decreases with increasing pH; increasing pH corresponding to an increase in N-MeIm concentration. However, if eq (11) (or (12)) could explain both the concentration profile data at pH 8.50 and the pH profile data at 0.800 M N-MeImT, then both sets of data should superimpose upon one another in a plot of kobs(2) versus N-MeIm concentration, which is clearly not the case as seen in Figure 6b. It therefore initially appears that one or more additional pH dependent equilibria must be added to the mechanism given in eq 10. Several possibilities were considered to explain the discrepancy between the concentration profile data at pH 8.50 and the pH profile data at 0.800 M N-MeImT. One possibility is that an R-hydroxo intermediate could form in alkaline solution; that is
The pKa for the R-aqua intermediate should be in the pH 6-10 range (since pKa(H2OCbl+) ) 7.6-8.112a,14d,20 and pKa’s(diaquacobinamide) ) 5.9 and 10.325). However, if the deprotonation of the R-aqua group of the intermediate was important in the pD range 6.5-9.5, since increasingly more of the R-hydroxo intermediate would be formed with increasing pD, less (N-MeIm)2Cbl+ would be formed. This would result in Kobs(2) decreasing with increasing pD in alkaline solution. From Figure 4, it can be seen that this does not occur; hence the formation of an R-hydroxo intermediate in the pH range of this study can be ruled out. On further thought, it becomes apparent that significant proportions of the R-hydroxo intermediate should only occur at much higher pH conditions, since the formation of the extremely stable base-on (N-MeIm)Cbl+ chelate ensures that the amounts of base-off R-aqua intermediate, (25) Marques, H. M.; Bradley, J. C.; Brown, K. L.; Brooks, H. Inorg. Chim. Acta 1993, 209, 161.
and hence base-off R-hydroxo intermediate, are minimal under the experimental conditions of this study. Similarly, the R-DMBI of the base-off aqua intermediate would be expected to protonate with a pKa ca. 5.5,14h but the formation of (N-MeIm)Cbl+ once again ensures that much more acidic pH conditions are required before a base-off DMBIH+ species is present in any significant amounts. The section entitled “Possible Intermediates” in the Supporting Information discusses the latter two points in more detail. The 1H NMR spectrum of HOCbl (0.100 M CAPS buffer, D2O, pD 10.5) was also recorded to see if there was any evidence for the existence of a base-off R-hydroxo complex for this cobalamin, HOCbl, in alkaline solution, pD 10.5. Five signals were observed in the aromatic region, consistent with the literature.17 No additional signals were observed. The possibility that the small discrepancies in the results are due to changes in the concentration of the cationic species contributing to the total ionic strength was also considered. Further experiments demonstrated that this cannot by itself be responsible for the deviation seen in Figure 6b, however, and are also discussed in more detail in the Supporting Information. The discrepancies may be due to a change in the mode of coordination of N-MeIm to the cobalamin upon acidification of the solution; that is, from nitrogen (N3) to carbon (C2) coordination.26 However, 1H NMR chemical shifts in the aromatic region for (N-MeIm)Cbl+ and (N-MeIm)2Cbl+ change by e0.03 and e0.10 ppm, respectively, upon going from pD 9.5 to pD 6.5. It was therefore considered unlikely that a change in the coordination mode of N-MeIm is occurring, though this cannot be totally ruled out. One explanation for the apparent inconsistencies seen in Figure 6b is that the effective concentration of N-MeIm available for binding to the R-aqua intermediate is pH-dependent, and decreases with decreasing pH. Perhaps an unproductive (i.e., unproductive in that it does not lead to product) ion-pair involving N-MeImH+ occurs, which leads to a decrease in the effective concentration of N-MeImH+ and hence N-MeIm also. The deviation between the data is very small; the difference between the two sets of results in Figure 6b is e30%. Alternatively, π stacking interactions between N-MeImH+ could be responsible for decreasing the effective concentration of N-MeIm available for binding.27 Activation parameters were obtained for the reaction (NMeIm)Cbl+ + N-MeIm a (N-MeIm)2Cbl+ at pH 8.50 at two (26) (a) Sundberg, R. J.; Martin, R. B. Chem. ReV. 1974, 74, 471 and references therein. (b) Tweedle, M. F.; Taube, H. Inorg. Chem. 1982, 21, 3361 and references therein. (c) Bonati, F.; Oro, L. A.; Pinillos, M. T.; Tejel, C.; Bovio, B. J. Organomet. Chem. 1994, 465, 267. (d) Lang, H.; Vittal, J. J.; Leung, P.-H. J. Chem. Soc, Dalton Trans. 1998, 2109. (27) References for intramolecular π stacking between the aromatic planes of imidazole ligands: (a) Carina, R. F.; Bernardinelli, G.; Williams, A. F. Angew. Chem., Int. Ed. Engl. 1993, 32, 1463. (b) Piguet, C.; Bernardinelli, G.; Williams, A. F. Inorg. Chem. 1989, 28, 2940. (c) Wilkes, J. S.; Zaworotko, M. J. Supramol. Chem. 1993, 1, 191. (d) Gelfi, C.; Mauri, D.; Perduca, M.; Stellwagen, N. C.; Righetti, P. G. Electrophoresis. 1998, 19, 1704. References for intramolecular π stacking between the aromatic planes of imidazole and another aromatic ligand: (e) Shimohigashi, Y.; Yamauchi, Y.; Ikesue, K.; Maeda, I.; Kawahara, M.; Nose, T. Pept. Sci.: Present Future, Proc. Int. Pept. Symp., 1st 1997, 649. (f) Kashima, A.; Inoue, Y.; Sugio, S.; Maeda, I.; Nose, T.; Shimohigashi, Y. Eur. J. Biochem. 1998, 255, 12. (g) Shao, C. Huaxue Yanjiu Yu Yingyong 1996, 8, 520. (h) Afshar, C.; Berman, H. M.; Sawzik, P.; Lessinger, L.; Lim, B. B.; Hosmane, R. S. J. Crystallogr. Spectrosc. Res. 1987, 17, 533. (i) Ishimaru, A. Bioorg. Chem. 1980, 9, 472. (j) Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97, 1303. (k) Mitchell, J. B. O.; Nandi, C. L.; McDonald, I. K.; Thornton, J. M.; Price, S. L. J. Mol. Biol. 1994, 239, 315. (l) Harata, K.; Muraki, M.; Jigami, Y. J. Mol. Biol. 1993, 233, 524.
Ligand Displacement at the R Axial Site of Cobalamins different N-MeImT conditions, 5.00 × 10-3 and 1.00 M N-MeImT. According to the mechanism given in eq 10, the rate constants obtained under these conditions correspond to the aquation of (N-MeIm)2Cbl+ and ring-opening in (N-MeIm)Cbl+, respectively. Large ∆Hq values are obtained for both reactions (99 ( 2 and 109.4 ( 0.8 kJ mol-1, respectively), corresponding to the breakage of the Co-N bond. The significantly positive values of ∆Sq (39 ( 5 and 70 ( 3 J mol-1 K-1, respectively) and ∆Vq (15.0 ( 0.7 and 16.8 ( 1.1 cm3 mol-1, respectively) clearly support the operation of a dissociative or dissociative interchange type of mechanism for both reactions. By way of comparison, ∆Vq values of 16.9 ( 0.8, 12.2 ( 0.5, and 10.0 ( 0.8 cm3 mol-1 were reported for the dissociation of the ligands (L) pyridine, 3-acetylpyridine and N,N′-dimethylthiourea from LCbl, respectively.12c The latter data were all interpreted in terms of a dissociatively activated substitution mechanism. Jencks and co-workers have examined the reaction between cyanocobalamin and cyanide in alkaline solution to form dicyanocobalamin, and on the basis of their results proposed the following mechanism:14d
Studies at two pH conditions (pH 9.0, 11.7) showed that the individual rate constants were independent of pH in this pH range; thus, even up to pH 11.7 insignificant amounts of an R-hydroxo base-off species are formed in this system. This mechanism is the same as our proposed mechanism, except that the reaction of the base-off aqua intermediate with cyanide to give cyanocobalamin is irreversible. In this study the rate constant for ring-opening of the intramolecularly coordinated R-DMBI of CNCbl to give the base-off R-aqua intermediate was found to be 4.2 × 10-2 s-1, which is 20 times larger than the corresponding rate constant for ring-opening of (N-MeIm)Cbl+ (2.08 × 10-3 s-1). This most likely arises from the known enhanced trans ligand effect of cyanide compared with NMeIm.7 Jencks points out that the ratio k-3/k4 represents the “effective molarity” for intramolecular addition of DMBI relative to the entering ligand. This ratio is small for both the CNCbl/(CN)2Cbl- (2.1 × 10-2 M) and (N-MeIm)Cbl+/ (N-MeIm)2Cbl+ (5.4 × 10-2 M) systems; thus the substitution of DMBI at the R axial site of the base-off R-aqua intermediate is not significantly enhanced by it being intramolecularly coordinated to the cobalamin. To summarize, the equilibria and kinetics of the reaction between (N-MeIm)Cbl+ and N-MeIm have been studied using 1H NMR and visible spectroscopies. The equilibria and kinetics of the reaction between H2OCbl+ and N-MeIm have also been reexamined. An unusual decrease in observed rate constant with increasing ligand concentration was observed for the reaction between (N-MeIm)Cbl+ and N-MeIm. The proposed mechanism involves formation of a base-off R-aqua intermediate prior to substitution of N-MeIm at the R site to form (N-MeIm)2Cbl+. While the concentration dependence data at pH 8.50 fit the proposed mechanism well, this is not the case for the pHdependent data at 0.800 M N-MeImT. The formation of an R-hydroxo intermediate or a base-off DMBI-protonated species
Inorganic Chemistry, Vol. 40, No. 7, 2001 1437 cannot account for this discrepancy, since insignificant amounts of these species exist in the pH range studied. π stacking interactions between N-MeImH+ may account for the deviations, however. Experimental Section Hydroxycobalamin hydrochloride (HOCbl‚HCl, >96%) was purchased from Fluka and N-methylimidazole (N-MeIm, g99%) from Aldrich. All solutions were made up in 0.100 M buffer (CAPS, CHES, TAPS, TES, BES, BIS-TRIS;28 Sigma) unless specifically stated otherwise and at a total ionic strength of 1.5 M (NaClO4, g98%, BDH). Distilled water was purified through a Milli-Q ultrapure water system. All pH measurements were carried out at 25.0 °C using an Orion Model 710A pH meter in conjunction with an Orion 9101 BN glass electrode and an Orion 900200 double junction reference electrode. The outer chamber of the reference electrode was filled with 1.60 M NH4NO3/0.200 M NaNO3, pH 7.0, which has been shown to have the same performance characteristics as the usual KCl filling solution. This was necessary since the solutions contained ClO4-. The electrodes were standardized using BDH 4.01, 6.98 and 10.00 buffer solutions; the latter buffer was freshly made up from a BDH pH 10 buffer capsule prior to use. Measurements in alkaline solution were carried out under a nitrogen or argon atmosphere. The error in individual pH measurements is no more than (0.01. 1H NMR spectra were recorded on an Inova 500 MHz NMR spectrometer equipped with a 5 mm thermostated (25.0 ( 0.2 °C) probe. All solutions were prepared in D2O and TSP was used as an internal standard. Visible spectra were recorded on a Cary 1E spectrophotometer equipped with a thermostated 8 × 6 cell changer and operated with WinUV Bio software (version 2.00). Unless stated specifically otherwise, all measurements were recorded at 25.0 ( 0.1 °C. Kinetic data at elevated pressures were recorded at 15.0 °C using a thermostated self-built high-pressure unit in conjunction with a Shimadzu UV2101PC spectrophotometer.29 An excellent fit to a single first-order rate equation was found for all kinetic data for at least 6 half-lives (OLIS-KINFIT fitting program30). All reported errors represent one standard deviation of the mean value. Acidity Constants. Acid Dissociation Constant of H2OCbl+. 40.00 mL of a HOCbl‚HCl solution (ca. 5 × 10-3 M (exact concentration known31), I ) 1.5 M (NaClO4)) was titrated with aliquots (0.100, 0.200, or 0.250 mL) of a NaOH solution (9.65 × 10-2 M, I ) 1.5 M (NaClO4)) under nitrogen at 25.0 °C. The experiment was done in duplicate. Acid Dissociation Constant of N-MeImH+. 25.00 mL of an N-MeIm solution (ca. 5 × 10-3 M (exact concentration known31), I ) 1.5 M (NaClO4)) was titrated with aliquots (0.100 or 0.200 mL) of a HClO4 solution (0.115 M, I ) 1.5 M (NaClO4)) under argon, 25.0 °C. The experiment was done in duplicate, with the N-MeIm used in the second titration being freshly vacuum distilled. Equilibrium Constant Determinations. H2OCbl+ + N-MeIm h (N-MeIm)Cbl+. Four separate sets of experiments were carried out at different pH conditions (pH 6.51, 6.99, 7.52, 7.98; 0.100 M buffer, (28) Abbreviations: CAPS ) (3-cyclohexylamino)-1-propanesulfonic acid, CHES ) (2-N-cyclohexylamino)ethanesulfonic acid, TAPS ) ((2hydroxy-1,1-bis(hydroxymethyl)ethyl)amino)-1-propanesulfonic acid, TES ) 2-((2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino)ethanesulfonic acid, BES ) N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid, BIS-TRIS ) 2-bis(2-hydroxyethyl)amino-2-(hydroxymethyl)-1,3-propanediol. (29) Spitzer, M.; Gartig, F.; van Eldik, R. ReV. Sci. Instrum. 1988, 59, 2092. (30) 1989 version; available from On-line Instrument Systems, Inc., Route 2, Box 111, Jefferson, GA 50549. (31) (a) The commercially available HOCbl‚HCl purchased from Fluka was found to contain 17 ( 2% H2O (by conversion to (CN)2Cbl(0.10 M KCN, 0.10 M phosphate buffer, pH 10.3; (CN)2Cbl- at 367 nm ) 3.04 × 104 mol-1 cm-1 31b). All HOCbl‚HCl concentrations were therefore corrected to allow for this. (b) Barker, H. A.; Smyth, R. D.; Weissbach, H.; Toohey, J. I.; Ladd, J. N.; Volcani, B. E. J. Biol. Chem. 1960, 235, 480.
1438 Inorganic Chemistry, Vol. 40, No. 7, 2001 I ) 1.5 M (NaClO4)). Solution mixtures containing a constant concentration of HOCbl‚HCl (ca. 5 × 10-5 M, exact concentration known31) and varying concentrations of N-MeImT (0-2.34 × 10-3 M, N-MeImT ) N-MeIm + N-MeImH+) were prepared in vials, capped and left to equilibrate overnight in a thermostated bath (25.0 ( 0.1 °C). The following day the absorbance of each solution was measured at 358 nm using the same cuvette (1 cm) for all measurements. (N-MeIm)Cbl+ + N-MeIm h (N-MeIm)2Cbl+. Seven separate sets of experiments were carried out at different pD conditions (pD 6.46, 7.00, 7.51, 8.00, 8.49, 8.98, 9.46; 0.100 M buffer, I ) 1.5 M (NaClO4)). Solution mixtures containing a known amount of HOCbl‚HCl (ca. 4.5 × 10-3 M, exact concentration known31) and varying concentrations of N-MeImT (5.00 × 10-2-1.20 M) were prepared in D2O in NMR tubes, capped and left to equilibrate overnight in a thermostated bath (25.0 ( 0.1 °C). The 1H NMR spectrum of each sample was recorded the following day and the relative proportions of (N-MeIm)Cbl+ versus (N-MeIm)2Cbl+ calculated from the areas of the (N-MeIm)Cbl+ and (N-MeIm)2Cbl+ signals at δ ) 5.88 ( 0.04 (β-A5) and 5.71 ( 0.05 ppm (β or R-A5), respectively. These signals were chosen since they are well separated from all other signals including those of the free (uncoordinated) N-MeIm(H+). Kinetic Measurements. H2OCbl+ + N-MeIm f (N-MeIm)Cbl+. Experiments were carried out at six different pH conditions for a range of N-MeImT concentrations (pH 6.01: [N-MeIm]T ) 8.00 × 10-41.50 × 10-2 M; pH 7.01: [N-MeIm]T ) 2.50 × 10-4-2.00 × 10-3 M; pH 7.30: [N-MeIm]T ) 3.13 × 10-4-1.80 × 10-3 M; pH 8.00: [N-MeIm]T ) 2.50 × 10-4-1.50 × 10-3 M; pH 8.50: [N-MeIm]T ) 3.13 × 10-4-1.80 × 10-3 M; pH 9.00: [N-MeIm]T ) 2.50 × 10-45.00 × 10-3 M; all in 0.100 M buffer, I ) 1.5 M (NaClO4)). In a typical experiment, 0.900 mL of a N-MeImT solution (1.38 × 10-33.00 × 10-2 M N-MeImT) and 0.900 mL of a ca. 1.00 × 10-4 M HOCbl‚HCl solution were mixed in a tandem cuvette and the absorbance monitored at 358 nm. For N-MeImT e 1.20 × 10-3 M, 0.450 mL of ca. 1.00 × 10-4 M HOCbl‚HCl solution and 0.450 mL of 0.100 M buffer solution were used instead of 0.900 mL of ca. 1.00 × 10-4 M HOCbl‚HCl solution. (N-MeIm)Cbl+ + N-MeIm h (N-MeIm)2Cbl+: Concentration Dependence Studies at pH 7.00 and 8.50. It was established early on that the observed rate constant, kobs(2), was independent of the cobalamin reactant used (solid HOCbl‚HCl or a solution of H2OCbl+ or (NMeIm)2Cbl+). For example, studying the rate of the reaction in 0.100 M N-MeImT, pH 8.50 using solid HOCbl‚HCl reactant gave 103kobs(2) ) 2.81, 2.88 and 2.99 s-1 (three separate experiments). Alternatively, when a solution of HOCbl‚HCl in 0.200 M N-MeIm was used (i.e., predominantly have (N-MeIm)2Cbl+), identical rate constants within experimental error were obtained (two separate experiments gave 103 kobs(2) ) 2.86 and 2.97 s-1). This N-MeImT concentration (0.100 M N-MeImT) and pH condition (pH 8.50) were specifically chosen to minimize the experimental error in the results (i.e., maximize the absorbance change for both reactants), since under these conditions the equilibrium mixture consists of a ca. 1:1 mixture of (N-MeIm)Cbl+ and (N-MeIm)2Cbl+. The fact that the observed rate constant is independent of the cobalamin reactant used is not unexpected, since the reaction was always studied under pseudo-first-order conditions and the formation of (N-
Cregan et al. MeIm)Cbl+ from H2OCbl+ and N-MeIm is rapid and irreversible under all conditions chosen to determine kobs(2). Solutions were therefore prepared so as to obtain the maximum absorbance change under the reaction conditions. Hence if (N-MeIm)2Cbl+ was the main cobalamin species at equilibrium, solid HOCbl‚HCl (or a solution of H2OCbl+) was used as the cobalamin reactant, whereas if (N-MeIm)Cbl+ was the main species, a solution of (N-MeIm)2Cbl+ (in an aqueous solution of N-MeIm) was used. Attempts were also made to isolate solid (N-MeIm)2Cbl+‚Cl- for use in kinetic studies. In a typical experiment 50 mg of HOCbl‚HCl was dissolved in a N-MeIm solution (0.40 mL, ca. 2 M, pH 8.5) and left to react at room temperature for ca. 2 h. Cold acetone was added (ca. 10 mL, 0 °C) to the solution and the solution left standing in a freezer (-20 °C) for 1 h. The resulting red product was collected and dried in a vacuum desiccator. The product was ca. 80% pure (i.e., 20% unwanted (N-MeIm)Cbl+ observed by 1H NMR spectroscopy), and it was suspected the product contained other impurities or even a small amount of N-MeIm since kobs(2) obtained using this material (0.100 M N-MeImT, pH 8.50) were substantially (ca. 20% larger) different from those obtained using either solid HOCbl‚HCl or a solution of HOCbl‚ HCl in 0.200 M N-MeIm. pH Dependence Studies. Seven pH conditions were investigated (pH 5.50, 6.49, 6.99, 7.49, 8.00, 8.50, 9.49). Ca. 4 µL of an H2OCbl+ solution (ca. 3.6 × 10-2 M) was added to a 1 cm cuvette that contained 3.00 mL of the appropriate solution (0.800 M N-MeImT, 0.100 M buffer (with the exception of the data collected at pH 9.49 in which no buffer was used), I ) 1.5 M (NaClO4)). Control experiments in the absence of buffer (CHES, TAPS, TES, BES or BIS-TRIS) gave identical results within experimental error (( 2%) with the exception of those obtained at pH 9.49 using CHES buffer. The latter results were found to be slightly dependent on the buffer concentration (103kobs (each value is the mean of 3 determinations) ) 2.16, 1.95, and 1.75 for 0, 0.100, and 0.200 M CHES buffer, respectively). The rate constants reported at pH 9.49 were therefore obtained in buffer-free solution. The absorbance was monitored at 358 nm.
Acknowledgment. The authors thank Ms. Maria Wolak, University of Erlangen-Nu¨rnberg, for the volumes of activation determinations and Mr. Chris Blake, Australian National University, for assistance with the NMR spectroscopy measurements. Financial support is gratefully acknowledged from the Deutsche Forschungsgemeinshaft and the Alexander von Humboldt Foundation. Supporting Information Available: Derivation of eq 2; Kobs(2) values at different pD conditions (Table A); kobs(2) values for [N-MeIm]T ) 5.00 × 10-3 M at pH 5.50, 7.00, and 8.50 (Table B); plots of Aobs versus [N-MeIm]T for equilibrated solutions of HOCbl‚HCl and N-MeImT at pH 6.99, 7.52, and 7.98 (Figures A-C, respectively); plots of kobs(1) versus [N-MeIm]T at pH 6.01, 7.01, 7.30, 8.00, 8.50, and 9.00 (Figure D-I, respectively); plot of kobs(2) versus N-MeIm at pH 7.00 (Figure J), plots of lnkobs(2) versus pressure at [N-MeIm]T ) 5.00 × 10-3 and 1.00 M (Figures K and L, respectively). This material is available free of charge via the Internet at http://pubs.acs.org. IC0009268
